Field of the Invention
The present invention relates to an ink jet printing apparatus and a method for controlling an ink jet printing apparatus for printing an image while moving a print head capable of ejecting ink.
Description of the Related Art
In an inkjet printing apparatus, if no ink has been ejected from nozzles of a print head for a long time, ink remaining near the nozzles may be dried or thickened, leading to defective ink ejection. In particular, a large-sized printer which prints images on wide print medium is likely to suffer from the situation in which no ink has been ejected from nozzles of a print head for along time. Correspondingly, such a printer is likely to be subjected to defective ink ejection.
Japanese Patent Laid-Open No. 2008-55855 describes a method for setting driving power (the ink ejection power of nozzles) for a print head according to the scan position of the print head in order to suppress defective ink ejection. The method involves detecting, based on image data to be printed, when the scan position of the print head changes from a no image area in which no image is printed to an image area in which an image is printed, and at the time of the detection, temporarily setting the driving power for the print head to a value larger than a regular value. Increasing the driving power of the print head in this manner allows ink to be temporarily intensely ejected so as to maintain an appropriate ink ejection state.
However, depending on a printed image, only some of the nozzles may not eject ink for a long time and may thus be caused to eject ink defectively. If some of the nozzles thus eject ink defectively, the density of the printed image may be uneven.
Japanese Patent Laid-Open No. 2008-55855 detects, for each unit area of the image data, the number of ink ejections corresponding to the area of the image data (the number of ejections corresponds to the number of dots formed by the ejected ink), and based on the number of ejections, determines whether the print area is an image area or a no image area. Furthermore, for the unit area of the image data subjected to determination, the length of the unit area in a sub-scan direction (which corresponds to the direction of the nozzles intersecting with a scan direction of the print head (main scan direction)) is set equal to the length of nozzle lines including all nozzles. Thus, it is difficult to detect when only some nozzles have not ejected ink for a long time. Furthermore, according to a multipass printing scheme of printing a predetermined print area by a plurality of scans carried out the print head, the image data is thinned out during the plurality of scans. Hence, with respect to a single nozzle, a position where ejection is started after no ink has been ejected for a long time may vary among the plurality of scans. Therefore, it is difficult to reliably suppress the defective ejection of ink from the nozzles simply by temporarily increasing the driving power for the print head when the no image area changes to the image area.